1. Field of the Invention
This invention relates to a surface fastener of thermoplastic resin having a flat substrate and a large number of hooks standing from the substrate as engaging members integrally molded and arranged continuously in array. The hooks of a surface fastener according to the invention hardly collapse, even if they are very small, and reliably come into engagement with mating loops of a matching surface fastener to provide a satisfactory engaging strength and peeling strength and a high ratio of engagement. A surface fastener according to the invention can endure repetitive engaging/disengaging operations and hence is particularly suitable for use in a paper diaper, a hospital gown, a napkin, worker's clothing, underwear or the like.
2. Description of the Related Art
Surface fasteners comprising a flat substrate and a large number of hooks integrally molded of molten thermoplastic resin by extrusion are disclosed in U.S. Pat. Nos. 4,984,339 and 5,441,687. In recent years, the demand for surface fasteners of this type have remarkably increased and they are currently used as fasteners in various applications including industrial materials, interior decoration articles for both cars and dwellings, daily necessaries and sanitary goods including paper diapers. This means that the engaging elements arranged on the substrate of a surface fastener may have to be dimensioned and profiled to meet the requirements of the application.
Meanwhile, as is understood from the above-identified U.S. Patents, known apparatus for manufacturing continuously and integrally molded surface fasteners are accompanied by a drawback that, due to its technical difficulty in molding process, the produced surface fastener does not provide a delicate touch of woven fabric and that, if hooks are formed with minimal dimensions, they do not provide a sufficient strength when they are simply molded to have a size like the monofilament hooks of known woven fabric type surface fasteners, so that they lack satisfactory durability. Additionally, with a hook structure integrally molded as described above, stems of the hooks have a simple cross-sectional profile and can become easily collapsed laterally or forwardly (or backwardly) particularly when they are made very small. Still additionally, engaging heads of the hooks have a simple profile and are too soft so that they do not provide a sufficient engaging strength and can easily be released from engagement. As a result, they can quickly lose the original profile after repeated use to significantly reduce the ratio of engagement with mating loops in a short time. Therefore, hooks have to be large if the rigidity and the engaging strength of the surface fastener are to be secured to a desired level. Then, such hooks become very rigid and only a limited number of hooks can be arranged per unit area (i.e. the density of hooks), and consequently they will no longer become effectively engaged with mating loops.
In an attempt to solve the above problems, WO 94/23610, U.S. Pat. No. 5,077,870, Japanese Patent Laid-Open Publication No. 2-5947 (U.S. Pat. No. 4,894,060) and Japanese Patent Laid-Open Publication No. 6-133808 disclose integrally molded surface fasteners having fine engaging elements.
Of the above listed patent documents, both WO 94/23610 and U.S. Pat. No. 5,077,870 propose a molded surface fastener having mushroom-like engaging elements arranged in place of ordinary conventional hooks. The mushroom-like engaging elements have an advantage in comparison with the engaging elements of hook-shape that they provide an enhanced effect of engaging mating loops and are easy to secure a desired level of engaging strength even if they are dimensionally reduced so that they are suitable for applications where the surface fastener is required to have flexibility. However, such a mushroom-like engaging element can easily become engaged with a number of loops particularly at the neck that connects the engaging head and the stem of the element so that the engaging element can be broken at the neck when it is forced to disengage itself from the loops regardless of its dimensions. Thus, a proposed surface fastener comprising mushroom-like engaging elements cannot withstand repeated use.
On the other hand, Japanese Patent Laid-Open Publication No. 2-5947 discloses a molded surface fastener that includes so-called J-shaped or palm tree shaped hooks standing from the substrate, which are included in the known general hook structures. However, the patent document proposes a disposable surface fastener by taking advantage that the surface fastener comprising such engaging elements can be manufactured at low cost and they can be engaged and disengaged with a mating non-woven cloth which can be manufactured at lower cost than a fibrous pily woven fabric. Thus, such a surface fastener may be applied to various disposable underwear and paper diapers. The surface fastener comprises fine engaging elements that are densely arranged on the substrate because the engaging elements are so small that each one of the engaging elements cannot provide a sufficient effect of holding a mating non-woven surface fastener, so as to secure the engaging strength and peeling resistance with the small pily fibers as a whole.
Japanese Patent Laid-Open Publication No. 6-133808 discloses a molded surface fastener in which engaging elements have T-shaped or inverted L-shaped front configuration. It specifically defines the height from the surface of the substrate to the lower end of the engaging head of each engaging element, the length of extension of the engaging head, the vertical thickness of the engaging head, the width of the engaging head, the projected surface area of the engaging head and the gap laterally separating adjacently located engaging elements in small numerical values, which are not different significantly from their counterparts of the surface fastener having small configuration disclosed in the preceding Japanese Patent document. However, the engaging elements have a specifically designed profile that differentiate them from any known engaging elements having fine engaging elements so that they provide a soft touch and can be engaged and disengaged smoothly from the mating loops, while securing a satisfactory engaging strength.
However, only by making the engaging elements small and dense or have a relatively simple profile, an enhanced engaging ratio with regard to the loops of a mating surface fastener may be realized, but they cannot necessarily secure an increased shear nor an increased peeling strength. Additionally, if such engaging elements are arranged quite densely, when the hook-shaped engaging heads of the engaging elements are forced into the soft fibrous loops of a matching non-woven surface fastener that are arranged also densely, either the fibrous loops or the engaging elements can collapse to make mutual engagement practically impossible and reduce the engaging ratio of the surface fasteners in comparison with the ordinary surface fastener.
Thus, the above-described molded surface fastener having very small engaging elements has a lower limit for the size of each engaging element and an upper limit for the density of arranging engaging elements. While Japanese Patent Laid-Open Publication No. 2-5947 defines the measurements of various parts, the meaning of the cited critical values are not necessarily clear. According to the above patent document, preferable values are between 70 and 100 pieces/cm2 for the density of arrangement of engaging elements, between 0.8 and 1.1 mm for the height of each engaging element, about 0.4 mm for the thickness of the stem and the engaging head of each engaging element (as viewed perpendicularly relative to the direction along which the engaging head extends), between 0.18 and 0.30 mm for the width of the stem (as viewed in the direction along which the engaging head extends) and between 0.25 and 0.37 mm or less than 1 mm for the length of the engaging head extending from the stem. These figures seem to be cited in order to secure the overall engaging effect both in the shearing direction and in the peeling direction because each engaging element have only an ordinary profile and no particular profile is devised for down-sized engaging elements so that a single engaging element shows a shear and an anti-peeling effect that are very low.
For the engaging head of an ordinary J-shaped engaging element of a surface fastener to be successfully forced into a loop of a mating surface fastener, the length of the engaging head as measured from a lower end to the top of the engaging head should be made as small as possible, while the distance between the lower end of the engaging head and the surface of the substrate and the distance between any two adjacently located engaging elements should be as large as several times of the effective width of mating loops. Thus, conventionally, the size of an engaging element are defined in terms of the width of the mating loop so that, if the engaging elements of the surface fastener are molded to be small and flexible in order to adapt them to a paper diaper, for example, the engaging head of each engaging element has to show an excessively arced profile to secure a necessary engaging strength and the distance between the lower distal end of the engaging head and the substrate which is necessary for letting the loop to be inserted therein has to be unequivocally defined.
This means that the height of each engaging element and the density of arranging engaging elements are unequivocally defined if a predetermined engaging ratio is to be secured and that, as a result, the height of each engaging element cannot be reduced any further. Therefore, assuming that the material of the molded surface fastener and the weight of each engaging element are invariable, the strength of the surface fastener both in the shearing direction and in the peeling direction cannot be improved unless the engaging elements are structurally modified and improved. Additionally, since the profile of the top of the engaging head of each engaging element that is standing from the surface of the substrate is curved, it is impossible to further improve the touch of the surface fastener and requires a relatively large mating loop in order for the engaging head to be successfully forced into the loop. If the small loops of the matching surface fastener are minimized, the engaging heads are prevented from entering in the loops. Still additionally, if the engaging elements of a surface fastener are simply down-sized without changing the profile, the hook-shaped engaging heads of the engaging elements are crushed to bend forwardly or laterally to make it further difficult for them to engage with matching loops and remarkably reduce the engaging ratio.
With a molded surface fastener as disclosed in Japanese Patent Laid-Open Publication No. 6-133808, it can be anticipated that each engaging element obviously shows only a reduced engaging strength with the mating loop if compared with the molded surface fasteners of the foregoing Publications. While it is seemingly intended to compensate the reduced engaging strength by devising a novel arrangement of engaging elements, the basic idea of securing the overall engaging strength of the surface fastener is practically same as that of the preceding patent document that proposes inverted J-shaped engaging elements, so that many various factors have to be defined in order to compensate the reduced engaging strength of each engaging element and secure the overall engaging strength of the surface fastener.
In the above described T-shaped or inverted L-shaped engaging elements of the cited patent document, the engaging head of each engaging element is not positively intended to have a thickness gradually decreasing from a base end to a distal end, but it is stated that the distal end portion is tapered. It merely describes that a projecting section of the engaging head has a thickness between 0.08 and 0.35 mm at the middle thereof. However, it does not describe any means for improving the rigidity of each engaging element and the proposed profile of each engaging element is not particularly inventive. Thus, it is easily understood that the thickness cannot be reduced to be less than 0.08 mm because the rigidity of each engaging element depends simply on that of the material.
Generally speaking, when the engaging elements of the surface fastener are down-sized, the substrate of the surface fastener have to be made thinner in order to make the surface fastener more flexible. However, if the substrate is made too thin, it can be extended unevenly or torn apart when the engaging elements of the surface fastener carried on it are drawn out of the mold at the end of the molding process to make the stable molding impossible. Even if they are drawn out successfully, the thinner the substrate be, the more undulations appear to reduce the market value of the product.
In view of the above identified various problems of known surface fasteners, it is therefore the object of the present invention to provide a molded surface fastener comprising, as integrally molded members thereof, a flat substrate and a large number of ingenuously profiled hooks standing from the substrate as engaging members and arranged continuously in rows such that the engaging elements can reliably engage with mating fine and dense fibrous loops of a matching non-woven surface fastener and hence the surface fastener shows an engaging strength, a shear and a peeling strength of a desired level, while providing a comfortable touch. The height of the engaging elements of such a surface fastener can be made smaller than that of the engaging elements of any known surface fasteners and hardly collapse, so as to maintain an enhanced ratio of engagement with mating loops. Such a surface fastener is durable and shows a desired level of flexibility and anti-tearing strength.